The present trend in semiconductor technology is toward large scale of integration of devices with very high speed and low power consumption performance. The parameters that are essential to such high performance bipolar transistors include the presence of low parasitic capacitances as realized by shallow vertical junctions and small horizontal geometries. In other words it is necessary to make the devices in the integrated circuit as small as possible in both the horizontal and vertical directions.
Most such high performance bipolar devices incorporate a sinker, that is a heavily doped region extending from the surface of the substrate through the top epitaxial layer down to the buried layer to provide a low resistivity connection from the N- collector of the epi-region to the low resistance N+ buried layer. The buried layer will thereafter provide the low resistance lateral contact which extends under the bipolar device. The purpose in any bipolar device is to minimize the collector resistance. In the above incorporated application, a cell design was disclosed which helped solve this problem by reducing the distance from emitter to collector to lower the lateral spacing and thereby the collector resistance.
However, it is still essential to minimize the vertical resistance from the surface of the substrate where the collector is diffused to the buried layer. If it were necessary to establish a path through the epitaxial layer to the buried layer, the overall resistance would be higher. Sinkers have been used in the past for this purpose but formed in a different way and forming a different structure, and having a different doping profile than in this invention.
The standard approach in the prior art is as shown in FIG. 1 wherein the collector and sinker are located in a region 2 separated by an oxide isolation 4 from the emitter and base generally indicated at 6 of the bipolar device. The sinker in such a structure is formed by cutting a hole in the oxide, doing a masking step and diffusing the N+ sinker 7 from the surface. The result is a sidewise diffusion as well as a deep diffusion, resulting in the devotion of considerable area (as illustrated in FIG. 1) to the collector and associated sinker.
As also shown in FIG. 1, the complete bipolar device is typically defined in a tub 8 edged by trenches 10, 12 which tub extends down through the epitaxial layer 14 and buried layer 16 to the substrate region 18. In the above incorporated applications, it was illustrated how to modify such a prior art approach to define the collector in the same tub as the emitter and base without the use of separating oxide regions. However, it is also desirable to efficiently incorporate a sinker in this structure to have an optimized bipolar device with minimum collector resistance.